The present invention relates generally to Long Band Optical Amplifiers with low noise figure. This invention is particularly suitable for use in long band optical amplifiers utilizing a rare earth doped fiber and at least one 1480 nm pump.
Optical amplifiers increase the amplitude of an optical wave through a process known as stimulated emission. During this process a pump photon, supplied by an optical pump, excites an electron to a higher energy level within an optical material. When a signal photon interacts with the excited electron, the electron undergoes a transition to a lower energy level. In the process, the material emits a coherent photon with the same frequency, direction and polarization as the initial signal photon. The two photons can, in turn, serve to stimulate the emission of two additional photons coherently, and so forth. The result is coherent light amplification. Stimulated emission occurs when the photon energy is nearly equal to the atomic transition energy difference. For this reason, the process produces amplification in one or more bands of frequencies determined by the atomic line width.
While there are a number of different optical amplifier configurations in use today, optical fiber amplifiers are quite popular, particularly for optical communications applications. The optical fiber amplifier typically includes an optical material such as glass, combined with a rare earth dopant such as such as Erbium and configured as an optical waveguide. Rare-earth-doped silica fibers are popular today in part because they offer the advantages of single-mode guided wave optics. Optical fiber amplifiers made of such fibers can be made to operate over a broad range of wavelengths, dictated by the atomic properties of the host and rare earth dopant. For example, Erbium doped fiber amplifiers (EDFAs) operate at two signal bands of the fiber transmission window. These signal bands are a conventional-band (C-band) with wavelength range from approximately 1528 nm to approximately 1565 nm and a long band (L-band) with wavelength range from approximately 1568 nm up to approximately 1620 nm.
In a typical optical amplifier fabricated using Erbium doped silica fiber, electrons are excited (pumped) from the ground state (4I15/2) to the metastable state (I13/2) by a pump at either 980 nm wavelength or 1480 nm wavelength. In the case of 980 nm pump, the electrons are first pumped to an excited state (4I11/2) and then non-radiactively decay into the metastable state (4I13/2) (See FIG. 1). In the case of 1480 nm pump the electrons are directly pumped into 4I13/2 state. The amplification occurs when the electrons in I13/2 state decay into the ground state by stimulated emission. After the electrons decay to the ground energy level 4I15/2, they can be re-pumped to the excited energy level 4I11/12 where they can take part in further stimulated emission processes.
Erbium doped fiber amplifiers (EDFAs) are typically made out of multiple stages of coiled Er-doped fibers. An example of such an Erbium doped fiber amplifier is shown in FIG. 2. The most critical parameters on the performance of EDFAs are noise figure (NF) and gain G. The noise figure, NF, measured in dBs is defined as 10times Log10 of the ratio of the signal (S) to noise (N) ratio at the input of the amplifier to that at the output of the amplifier. That is, NF=10xc3x97Log10[(S/N) in/ (S/N) out]. The gain, G, is defined as the ratio of signal output power to signal input power. In multistage amplifiers the noise figure NF is largely determined by the gain G in the front end of the amplifier. Thus, the higher the gain G of the first coil of the EDFA, the lower NF. Another measure of EDFA performance is the power efficiency, which is defined as the ratio between the number of signal photons amplified and emitted to the number of pump photons. Since the performance of the communication system is determined by the noise performance of the amplifiers in the system, signal power of the amplifiers, and fiber transmission properties, optical communication systems require that the EDFAs have the lowest possible noise figure (NF), and provide the highest possible gain, (G).
FIG. 3A illustrates the absorption spectrum of the Erbium doped fiber (EDF). This figure shows that a strong absorption peak is present in the 980 nm-pumping band. Because of the strong absorption at the 980 nm wavelength, some long-band EDFAs use a 980 nm pump in conjunction with a first EDF coil. The use of the 980 nm wavelength pump introduces high inversion at the front end of the optical amplifier, thus resulting in a relatively low noise figure. (See FIG. 2.) The 980 nm pump provides less efficient power conversion relative to 1480 nm pump and is relatively difficult to build. Therefore, a 980 nm pump is expensive. However, it is a common belief that the use of a less efficient, more expensive 980 nm pump (as the first pump in the L-band amplifier) is needed in order to provide low noise and thus, high signal to noise ratio so that a cleaner signal is provided to the second EDF coil for further amplification.
The major degradation to NF in L-band EDFAs comes from BASE (backward amplified spontaneous emission). The BASE accumulates over long fiber length and is due to high inversion at the front end of the optical amplifier. The strong BASE saturates the front end inversion, thus reducing the gain at the front end and increasing the NF of the amplifier. It is known that a spectral filter can be used to block BASE in the 1528 to 1565 nm wavelength range resulting in NF improvement of about 0.5 dB. However, the spectral filter is complex and relatively difficult to manufacture, which makes it expensive. In addition, the spectral filter does not function as an ideal step function and affects the transmission T in the signal band, for example, through the oscillatory ripple. (See FIG. 3B) This will introduce loss in the gain spectrum, known as gain ripple, of about 0.3 dB. Finally, spectral filters introduce a relatively large insertion loss, which often is about 0.6 dB.
In order to extract maximum power, the rear stage pump (i.e., the pump that is coupled to the second EDF coil) is typically a more efficient, less expensive to manufacture, 1480 nm wavelength pump (see FIG. 2). It is known that this second pump will improve the efficiency of the multiple stage EDF amplifier without introducing too much noise into the system.
It is desirable to provide low noise L-band optical amplifiers that are also more efficient than prior art long band optical amplifiers.
According to one aspect of the present invention an optical amplifier comprises: a first gain medium having an optical host that contains a rare earth dopant and a first pump that supplies optical energy at a first wavelength into this first gain medium. The first gain medium is an optical fiber of a predetermined length, such that inversion of this first gain medium is not saturated. The optical amplifier further comprises a second gain medium operatively coupled to the first gain medium and a second pump that supplies optical energy into the second gain medium.
According to an embodiment of the present invention the predetermined length of the optical fiber of the first gain medium is 1 to 6 meters and preferably 1.5 to 4.5 meters.
More specifically, according to one embodiment of the present invention a optical amplifier includes:
(i) a first gain medium having an optical host that contains a rare earth dopant, the first gain medium being an Er doped optical fiber having a length of less than 10 meters,
(ii) a 1480 nm first pump that supplies optical energy at a first wavelength into the first gain medium;
(iii) a second gain medium operatively coupled to the first gain medium, the second gain medium being an Er doped optical fiber having a length of greater than 10 meters; and
(iv) a second pump supplying optical energy into the second gain medium.
Embodiments of the present invention can provide an optical amplifier and a pumping technique that overcomes the difficulties associated with prior art L-band optical amplifiers. It is an advantage of this optical amplifier that it has low noise and high pumping efficiency.
For a more complete understanding of the invention, its objects and advantages, refer to the following specification and to the accompanying drawings. Additional features and advantages of the invention are set forth in the detailed description, which follows.
It should be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.